linux-kernel - Re: [PATCH 1/2] uapi: fcntl: guard AT_RENAME

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Message-ID: <20251109161342.497e7e7e@pumpkin>
Date: Sun, 9 Nov 2025 16:13:42 +0000
From: David Laight <david.laight.linux@...il.com>
To: Masaharu Noguchi <nogunix@...il.com>
Cc: jesperjuhl76@...il.com, Jeff Layton <jlayton@...nel.org>, Chuck Lever
 <chuck.lever@...cle.com>, Alexander Aring <alex.aring@...il.com>,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] uapi: fcntl: guard AT_RENAME_* aliases

On Sun,  9 Nov 2025 16:13:03 +0900
Masaharu Noguchi <nogunix@...il.com> wrote:

> Signed-off-by: Masaharu Noguchi <nogunix@...il.com>
> ---
>  include/uapi/linux/fcntl.h | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> index 3741ea1b73d8..e3026381fbe7 100644
> --- a/include/uapi/linux/fcntl.h
> +++ b/include/uapi/linux/fcntl.h
> @@ -157,9 +157,15 @@
>   */
>  
>  /* Flags for renameat2(2) (must match legacy RENAME_* flags). */
> +#ifndef AT_RENAME_NOREPLACE
>  #define AT_RENAME_NOREPLACE	0x0001
> +#endif
> +#ifndef AT_RENAME_EXCHANGE
>  #define AT_RENAME_EXCHANGE	0x0002
> +#endif
> +#ifndef AT_RENAME_WHITEOUT
>  #define AT_RENAME_WHITEOUT	0x0004
> +#endif

That looks like the start of an 'accident waiting to happen'.
Either #undef the constants so that code is guaranteed to get the 'uapi' values,
or do explicit checks, eg:

/* stdio.h may have defined AT_RENAME_NOREPLACE */
#if !defined(AT_RENAME_NOREPLACE) || AT_RENAME_NOREPLACE != 0x0001
#define AT_RENAME_NOREPLACE 0x0001
#endif

	David


>  
>  /* Flag for faccessat(2). */
>  #define AT_EACCESS		0x200	/* Test access permitted for